Many multi-axis, multi-sensor coordinate measurement machines typically use crash-detection or crash-prevention mechanisms to avoid potential damage to probes and other sensing devices. Most of these mechanisms employ a release mount of the probe and/or sensors when a predetermined amount of force is applied to the probe and/or sensor. Because this force may include an impact element, the probe and/or sensors can be thrown out of alignment, requiring realignment and recalibration of the probe/sensors and the Z-axis at significant cost in time and funds. To reduce the amount of realignment and recalibration required after a collision, others have pursued various arrangements.
Consider, for example, U.S. Pat. No. 6,852,002 to Stewart et al., assigned to Flow International Corporation and entitled, “Apparatus and Methods for Z-Axis Control and Collision Detection and Recovery for Waterjet Cutting Systems.” The cutting system includes a linear rail, a slide member coupleable to a cutting head and slidably coupled to the linear rail, an actuator having coupled to the slide member and fixed to the linear rail, a position sensor, and a controller. The actuator provides an adjustable support force that supports the weight of the cutting head, allowing the cutting head to be controllably positioned at a desired height above the workpiece. Stewart et al. use a first mount member coupleable to a controllably positionable mounting surface of the cutting system, a second mount member coupleable to the cutting head and disengageably coupled to the first mount member, and a sensing circuit having a plurality of first conductive elements disposed on the first mount member and a plurality of second conductive elements disposed on the second mount member. If the cutting head collides with an obstruction, the second mount member disengages from the first mount member to prevent breakage of the cutting head. After a collision, the second mount member is re-engaged with the first mount member without recalibration. Re-engagement of the second and first mount members can be performed automatically by a biasing member. While this is a step in the right direction, the arrangement can result in movement of the tool out of its aligned position. When the tool is reconnected, the degree to which it returns to its original alignment, and its repeatability, is not as high as a high precision metrological instrument requires.
Also consider U.S. Pat. No. 5,867,916 to Matzkovits, assigned to Carl-Zeiss-Stiftung and entitled, “Coordinate Measuring Machine with Collision Protection.” This system is a coordinate measuring machine with a measuring arm on which a collision protector is provided. The collision protector can be deflected transversely of the longitudinal axis of the measuring arm when the measuring sensor system collides with an object. To operate the coordinate measuring machine with different measuring sensor systems and machining units, the coordinate measuring machine includes an identification unit that automatically identifies the measuring sensor system or machining unit. A securing unit is connected to the identification unit and allows adjustment of the torque required to deflect the collision protector in response to identification of the measuring sensor system or machining unit by the identification unit. While this prevents damage to the sensing unit, the collision protector is a breakaway portion of the measuring arm. When a collision induces movement of the collision protector, the arrangement does not guarantee precise realignment when the collision protector returns to its original position.
U.S. Pat. No. 5,210,399 to Maag et al., assigned to Carl-Zeiss-Stiftung, and entitled, “Optical Probe Head with Mounting Means Providing a Free Recalibration of the Sensing Head after a Collision,” keeps all position-sensitive components rigidly fixed using a design similar to that of Stewart et al. An optical probe head has a front optic and an annular enclosure surrounding the front optic. The enclosure contains the illuminating device of the probe head. The front optic is rigidly attached to the optical probe head and the enclosure having the illuminating device and surrounding the front optic is attached to the optical probe head so as to be radially yieldable, such as with bearings related to the pin and ball arrangement of Stewart et al. In the case of a collision, only the enclosure having the illuminating optics is deflected, the imaging optics remaining undisturbed. In this way, Maag et al. state that a follow-up calibration of the probe head after a collision is no longer required.
DE19622987 to Mettendorf et al., assigned to Mycrona, and entitled, “Collision Protection Appliance for Sensors on Coordinate Measurement Machine.” The appliance has a laminar clearance sensor (2) on the lower end of its measurement sensor (1). The sensor can be a capacitive device with its beam lobe directed both radially and axially. The beam lobe of the capacitive sensor can be directed radially and the axial protection against collision can be provided by a ring suspended from a mechanical switch.
Embodiments solve this dilemma of realignment and calibration of the vertical axis and the primary measurement sensor, as well as secondary and tertiary measurement sensors, if present, by removing all collision-related release from the instrument tower. A mounting plate to which a fixturing device, such as a rotary module, can be attached rests on a base plate via a kinematic mount arrangement. The kinematic mount of embodiments allows the mounting plate to break away from the base plate in the event of a collision, yet provides enough resistance that ordinary operative fluctuations in moment and orientation of the mounting plate resulting from motion of the fixturing device do not initiate breakaway. Additionally, the kinematic mount allows the mounting plate to be replaced in the kinematic mount to within microns of its original position after a collision, eliminating the need for recalibration of the instrumentation. If, however, calibration is required, a simple, quick calibration can be performed using a removable artifact.
Additionally, embodiments employ a crash detection system, preferably mounted on the mounting plate. The crash detection system of embodiments uses sensors, such as proximity sensors, and a controller to monitor the state of the mounting plate and, when the mounting plate breaks away from the base plate, stops the machine in which the breakaway/crash detection system is used. By making the breakaway/crash detection system part of the portion of the device that holds an object to be inspected, the sensors are isolated from shift due to a collision and thus do not need to be recalibrated after a collision/breakaway. Instead, the plate can simply be replaced on the base plate with no calibration, and the inspection can be restarted or resumed. If calibration is required, a very quick procedure can be employed involving a reference artifact placed on the plate. Thus, embodiments eliminate the need for realignment and calibration of sensors after a collision.
The rotary module, or other fixturing devices, to which the breakaway/crash detection system is fixed, breaks away from the solid horizontal axis of motion in embodiments upon collision. The break away design allows the optical system, probes, sensors, and any other position-sensitive components to be rigidly mounted to the vertical axis of the machine and minimizes or eliminates the release and re-align problem. This preserves the accuracy and repeatability of the optics, sensors, probes, and axis of motion in the event of a collision. Additionally, by placing the breakaway design low on the horizontal axis, the possibility of any small errors accumulating during re-alignment is reduced, particularly in accordance with vertical axis. Such small errors are amplified as the focal or working distance of the sensors is increased. The break away design of embodiments thus overcomes the alignment and calibration issues present in the prior art.